Redshift Parameter

1. The problem statement, all variables and given/known data
The cosmological principle can also be used to derive an expression for the redshift of light
(from far away galaxies) in terms of the scale factors at emission and observation. Use the fact
that the wavelength in terms of the co-moving coordinates Δx is independent of time to derive an expression for in terms of the scale factors a(tobs) and a(tem). The redshift parameter z is the typical parameter used to indicate distance in cosmology.

(I suppose it is obvious, but obs stands for observer and em for emitter)

2. Relevant equations

3. The attempt at a solution
To be honest, I do not even know where to begin with this. I have looked over http://en.wikipedia.org/wiki/Redshift#Mathematical_derivation but that seems far more complicated than anything we have been doing so far. I suppose I have to utilize one of the Friedmann equations, but which one in what way I don't know. I understand that this forum isn't a solve my homework for me forum, so what I'm asking for is a push in the right direction, so I can start working on this.